Generally, a golf club comprises a shaft portion, a head portion, and a grip portion. That part of the golf club head portion which comprises a hitting surface is called a golf club face. See, e.g., R. Maltby, "Golf Club Design, Fitting, Alteration & Repair" (4th Ed. 1995). Generally, a golf club face abuts or is adjacent to both a top wall or crown of the club head and a bottom wall or sole of the club head. Typically, "crown" and "sole" are used to designate the top portion and bottom portion of a wood-type club head, and "top wall" and "bottom wall" are used to designate the top portion and bottom portion of an iron-type club head as well as a wood-type club head, and will be so used throughout the present specification.
In both wood-type club heads (which today are typically hollow, but are not necessarily so (e.g., may be foam filled)) and cavity back iron-type club heads the golf club faces are preferably thin. Such golf club faces generally define two surfaces: a hitting surface (i.e. front side) and a surface which is opposite the hitting surface (i.e. a backside).
When the front side of a face of a golf club head strikes a golf ball, large impact forces (e.g. up to 2000 pounds) are produced. These large impact forces load the club face. In the relatively thin faces of wood-type club heads and cavity back iron-type club heads these forces tend to produce large internal loads, such as, for example, large bending stresses. These internal loads may cause catastrophic material cracking which causes the club head to be unusable. The phrase "internal load" is used throughout the present specification to refer to, for example, the bending moments, shear forces, and axial forces experienced by a golf club face as the result of an applied force (e.g. at least one ball impact).
Recent computational and experimental studies on wood-type club heads and cavity back iron-type club heads have shown that catastrophic material cracking due to large internal loads (i.e. due to an applied force) most often occurs in at least one of the following three face locations: (1) in the club face hitting surface (front side) at the ball strike center which is an area of large compressive bending stresses, particularly in the area of any score-lines; (2) in the club face back surface (back side) at the ball strike center which is an area of large tensile bending stresses; and (3) (a) at the portion of the intersection of the face and the top wall which lies directly above the ball strike center which is an area of a large vertical component of the bending stresses, and/or (b) the intersection of the face and the bottom wall which lies directly below the ball strike center which also is an area of a large vertical component of the bending stresses. The area between the face/top wall intersection region (i.e. where the face and top wall meet) above the ball strike center and the face/bottom wall intersection region (i.e. where the face and bottom wall meet) below the ball strike center may be called a ball strike zone.
It has also been found that the vertical stress distribution through the ball strike zone on the back side of the face comprises large compressive (i.e. negative) applied stresses in the face/bottom wall intersection region which increase to zero toward the ball strike center region, reach a maximum tension (i.e. positive) value behind the ball strike center region, and decrease through zero to large compressive (i.e. negative) applied stresses toward the face/top wall intersection region. The vertical stress distribution through the ball strike zone on the front side (or hitting surface) of the face generally has the same, but opposite, components (i.e. large tension bending applied stresses at face/bottom wall intersection which decrease to large compressive applied stresses at ball strike center and then increase to large tension bending applied stresses at face/top wall intersection).
In designing golf club heads, the golf club face portion must be structurally adequate to withstand large repeated forces such as those associated with ball impact. Such structural adequacy may be achieved by increasing the face portion stiffness so that the load produced internal stresses are below the critical stress levels of the material used in the face. Typically, the face portions of club heads are stiffened by uniformly increasing the thickness of the face portion and/or by adding one or more ribs (i.e. discrete attached posts or lines) to the back surface of the face.
Uniformly increasing the thickness of the face portion typically requires the addition of a large amount of material to adequately reduce the internal load sufficient to prevent impact and/or fatigue cracking (i.e. to adequately withstand or tolerate the applied force, e.g., ball impact). However, the addition of such a large amount of material to a club face generally adversely affects the performance of a club incorporating such a face. The club performance is adversely affected by the overly heavy club face. In addition, the feel of a club incorporating such a face is also adversely affected by the large number of vibrations transmitted through the club. Furthermore, if a maximum club head weight were imposed, a face with added material prohibits distribution of that weight to other areas of the head where it may be preferred (i.e. more weight on the face means less weight, for example, along the perimeter of a cavity back iron-type club head).
Adding ribs to the back surface of a face to stiffen the face has the benefit of stiffening without adding a significant amount of weight to the face, but has the detrimental result of creating an irregular stiffness distribution on the face hitting surface. Examples of ribs which have been used in prior golf club head designs include, for example, vertical ribs, horizontal ribs, curved ribs, dendritic ribs, angled or skewed (i.e. V or X patterned) ribs, circular ribs, or a combination of more than one of these types. Such ribs are generally geometrically characterized as having a narrow width, any desired length, and a sufficient depth or thickness to locally increase the face stiffness and yet minimize the increase in face weight.
In addition, such ribs are typically shaped such that a sharp corner (or a curved corner with a small radii) is formed between a rib and the face back surface where the rib is attached. Such corners lead to cracking potential as they create stress focus points. Furthermore, the use of ribs which are positioned to run vertically along the face back surface cause the large bending applied stresses (which were described above) to travel to the face/bottom wall intersection and face/top wall intersection thereby increasing cracking at those positions.
Additional problems experienced with the use of ribs on a face back surface are in the manufacture of such faces. Typically faces are formed using a casting process. It is more difficult to cast faces which include rib structures due to nonuniform material shrinkage which occurs during cool-down of such a casting. Such non-uniform cool-downs tend to cause inclusions, internal voids, and/or surface cracking in the cast materials, particularly along regions where ribs are positioned. Such non-uniform cool-downs also tend to cause face depressions and surface dimpling in the hitting surface opposite the regions where ribs are positioned.
Thus, there is a need for a new club face structure with increased structural integrity (and, thereby, reduced cracking and material failure) without adversely affecting club performance, look, and feel; and with limited affect on desired club head weight distribution.